Blind fasteners, or rivets, as well known in the art, are utilized in fastening components in which only one side of the workpiece is accessible. U.S. Pat. No. 3,148,578 to Gapp describes a fastener assembly comprising a fastener body in the form of a sleeve, and an elongated stem within the fastener body. One end of the stem has serrations for engagement by a pulling tool. The opposite end of the stem has an upsetting head. Adjacent the tail end of the sleeve is a shear ring, integrally formed with the stem.
In operation the Gapp assembly is inserted through aligned holes in juxtaposed workpieces to be secured together. Once fully inserted, a tool is used to pull the stem axially away from the workpiece, while a reactionary force is applied against a head of the sleeve. During this first part of the pulling operation, the shear ring causes radial expansion of the tail end of the sleeve and bulbing on the blind side of the workpieces. The shear ring is constructed to shear from the stem at a predetermined tension on the stem, thereby limiting the amount of clamping compression on the workpieces to a predetermined value. Continued pulling causes a lock ring groove in the stem to reach the sleeve head where a lock ring is deformed into the groove. Pulling increases the load, causing the stem extending beyond the accessible face of the workpieces to break away.
Shear rings integrally formed with the mandrel stem suffer from many manufacturing disadvantages. For example, cutting tools are typically used to form a shear ring in a blank of material. Such machining is not only time consuming, but also produces a large amount of scrap. Furthermore, as the cutting tools are used they become dull and worn, and slight variations in the dimensions of the shear ring tend to appear. Other manufacturing processes such as heading or coining are possible, but it is difficult to hold adequate tolerances so that consistent shearing loads are obtained.
A blind fastener utilizing a separate shear ring is shown in U.S. Pat. Nos. 5,052,870, 5,056,973, and 5,131,107 issued to Pratt. These patents disclose a blind fastener having an elongated stem with an enlarged blind end adjacent a cylindrical neck, with an annular groove machined in the neck. A tubular fastener body receives the stem with the neck fitting closely within a tail end of the body. An anchor portion of a shear ring fits in the annular groove with a bulbing portion of the ring extending radially beyond the stem neck. When the blind end of the stem is drawn toward the fastener body, the bulbing portion of the shear ring forms a blind head on the fastener body; and when a predetermined stem pulling load is reached, the bulbing portion of the ring is sheared from the anchor portion.
Advantageously, the Pratt patents utilize a shear ring extruded as a shaped wire that is wrapped around the fastener stem, with the anchor portion fitting within the annular groove. Utilizing the extrusion process, the shaped wire can be formed relatively inexpensively with dependable tolerances; thus, there is less variation in the axial dimension of the shear ring, resulting in a very reliable prediction of the forces required to shear the ring. Because the shear ring is not integral with the stem, its composition is not limited to the material used to form the stem. This is particularly beneficial from a manufacturing standpoint in that a single lot of mandrel stems can be wrapped with any one of a number of precise shear ring wires to produce a wide variety of fasteners. Unfortunately, the process of wrapping a relatively large cross section shaped wire around the mandrel stem inevitably leads to a slight amount of springback of the wire. This springback results in a discontinuity in the shear ring around the mandrel stem, exposing sharp edges and possibly causing rupture of the bulbing tail of the fastener body. One solution to this problem is to band anneal the sleeve tail so there is less tendency to split; however, this adds a manufacturing step. Also, after a fastener has been installed with a split expander there can be some tendency for the sheared expander or bulbing portion to move rearwardly, enlarging its diameter and resulting in less holding force on the workpiece.
Thus, there exists a need for an improved shear ring for blind fasteners that will overcome the deficiencies of prior designs.